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Of course it varies and if you want a link with a mind-numbing list like the end of a drug commercial, I've added one below. However to be more helpful I will characterize the side effects into major effects for most people. All of this multiplies in the face of other stimulants including caffeine.

Psychological
Though modafinil can initially be somewhat euphoric and social, it is inevitably neither. Regular use is likely to make you impatient and easily irritated.

Pulmonary
Modafinil can increase or throw off your heart beat. If you have any pathology here it could be a serious matter; stay away, it's not a well researched area yet. Otherwise don't freak out, just dose down at the first sign of a rapid or trembling heart beat..

Sex
A helper for neither foreplay nor ejaculation.

Coordination
Modafinil will not win any Olympic Medals, it builds a rhythm suitable to concentration, not athletic flow.

Cramps
Modafanil can cause fingers and toes to lock up, a very scary phenomena at first yet not shown to be anything particularly life threatening. Take it as a sign to back off and consider alternative nootropics.

A large population will experience none of this, don't be surprised if you are not experiencing side effects and don't let their absence be a rationale for over use.
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